I tend to agree with what 5twins is saying. I'm sure a hitch pin is more that safe for the rear axle. Even in the remote chance that the pin came out, the axle nut is not going to loosen.
I use a regular cotter pin on my rear axle. Whenever I remove the cotter pin, it always just comes out with my fingers. In other words, the nut has never loosened enough to put pressure on the cotter pin. In my experience, large nuts like on the rear axle do not loosen up by themselves. Once the nut is tight, there is no rotational force large enough to loosen the nut.
The only bolts/nuts I've noticed that back off are 6 mm size threads or smaller.
Having said that, I will always use a cotter pin where there are castlelated nuts. To me that is just safety in depth logic. While I believe it would be very rare for an axle nut to loosen, the cotter pin gives a second barrier.
BMW is obviously very confident that axle nuts will stay tight.